The present invention relates to a process for producing hollow extrudates for use in a vacuum, and more particularly to a process for producing a hollow extrudate for use in a high vacuum such as a pipe providing a particle accelerating beam chamber.
While stainless steel pipes were used for particle accelerating beam chambers, aluminum alloys have recently been found to be suited to this application and therefore placed into use. Such pipes defining a particle accelerating beam chamber are usually prepared from a billet of AA6063, AA6061 or like Al-Mg-Si alloy by a process comprising the steps of hermetically closing the forward open end of a hollow shaped material immediately after extrusion from the billet, subsequently extruding a predetermined length of shaped material, cutting off the predetermined length of extruded material and hermetically closing the cut end thereof at the same time, and cutting off the opposite closed ends (see U.S. Pat. No. 4,578,973).
With this conventional process, the inner surface of the hollow portion of the shaped material is substantially held out of contact with the atmosphere during extrusion, consequently preventing formation on the inner surface of a hydrous oxide film which is liable to adsorb or occlude vacuum reducing substances and alternatively permitting formation of an oxide film which is compacter and thinner than the hydrous oxide film. The latter oxide film is much less likely to adsorb or occlude vacuum reducing substances, and such substances, even if adsorbed or occluded, are easily removable by an outgassing treatment. Accordingly, the chamber can be maintained at a high degree of vacuum, with greatly reduced quantities only of objectionable substances released into the chamber. This serves to obviate or lessen the cumbersome work that would otherwise be needed to afford a high vacuum.
The billet conventionally used in the above process for preparing extrudates is made of AA6061, AA6063 or like Al-Mg-Si alloy in view of extrudability and mechanical strength. However, such a material contains Mg, etc. diffused through the surface oxide film, so that the film is less compact than the oxide film of Al only to be formed on a material of pure aluminum. It would then appear preferable to use pure aluminum for preparing extrudates, but the use of pure aluminum which has lower strength than Al-Mg-Si alloys entails the necessity of giving an excessively increased wall thickness to the extrudate to assure the strength required of the product.
Further when a billet of Al-Mg-Si alloy, such as AA6063 or AA6061, is used for preparing a hollow extrudate for a particle accelerating beam chamber, the following problem arises owing to the fact that the alloy fails to exhibit sufficiently reduced electrical resistance at extremely low temperatures such as the temperature of liquid helium. Presently, the beam traveled in the chamber has energy of about 30.times.30 GeV, and the current flowing along the inner wall surface of the chamber is not very great, so that the beam is unlikely to become unstable even if the electric resistance of the alloy is not sufficiently low at very low temperatures. Nevertheless, it is attempted to use a beam for travel in the chamber with increased energy of about 20000.times.20000 GeV. If the beam energy is increased to such a level, the wall current also increases, with the result that the use of AA6063 or AA6061 alloy renders the beam unstable because the electrical resistance of the alloy is not sufficiently low at extremely low temperatures.